Method of generating hydrogen from the reaction of stabilized aluminum nanoparticles with water and method of forming stabilized aluminum nanoparticles

ABSTRACT

A method of generating hydrogen gas from the reaction of stabilized aluminum nanoparticles with water is provided. The stabilized aluminum nanoparticles are synthesized from decomposition of an alane precursor in the presence of a catalyst and an organic passivation agent, and exhibit stability in air and solvents but are reactive with water. The reaction of the aluminum nanoparticles with water produces a hydrogen yield of at least 85%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/791,900, filed Jun. 2, 2010, which claims the benefit of U.S.Provisional Application No. 61/183,229, filed Jun. 2, 2009, entitledGENERATION OF HYDROGEN FROM REACTION OF ALUMINUM-ORGANIC CORE SHELLNANOPARTICLES AND WATER. The entire contents of said applications arehereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Contract No.F33615-03-02-2347 awarded by the Air Force; Contract No. HDTRA-007-0026awarded by the Defense Treat Reduction Agency; and Contract No.RHS-UD-08-02, awarded by the Dayton Area Graduate Studies Institute. Thegovernment has certain rights in the invention.

BACKGROUND

Various embodiments of the invention relate to the synthesis ofstabilized aluminum nanoparticles, and more particularly, to a method ofproducing hydrogen from the reaction of stabilized aluminumnanoparticles with water.

The desire for the development of technologies which utilize hydrogengas for power generation has increased in recent years. However, the useof hydrogen for such applications requires readily available, safe, andenvironmentally friendly access to hydrogen.

It is known that the reaction of aluminum with water yields hydrogengas:

2Al+6H₂O→2Al(OH)₃+3H₂  (1)

2Al+4H₂O→2ALO(OH)+3H₂  (2)

2Al+3H₂O→Al₂O₃+3H₂  (3)

However, these reactions are limited due to the natural occurrence of aprotective aluminum oxide shell on the surface of the aluminum. Thestability of aluminum oxide prevents air and moisture from accessing theunderlying aluminum metal.

To address these problems and facilitate the generation of hydrogen,various attempts have been made to alter the reaction, such as theaddition of strong bases, the application of high temperatures, oractivation of the aluminum metal. For example, it has been found that bydissolving aluminum in liquid gallium, the formation of the aluminumoxide shell is prevented, thus allowing the aluminum-water reaction toproceed. However, the procedure for such a reaction is complex.

SUMMARY OF THE INVENTION

It is against the above background that embodiments of the inventionalter the nature of the protective aluminum oxide shell in a mannerwhich allows the aluminum-water reaction to proceed easily and safely togenerate large quantities of hydrogen.

Embodiments of the invention provide a method of synthesizing aluminumnanoparticles which are stable in air and solvents, but which arereactive with water and can be used to generate hydrogen. The stabilizedaluminum nanoparticles are formed by decomposition of an alane precursorin the presence of a catalyst and an organic passivation agent such thatthe formed nanoparticles have an organic outer shell and anorganic-provided oxide inner shell. The reaction of the stabilizedaluminum nanoparticles with water is spontaneous and requires nopromoters or energy to initiate the reaction, and the hydrogen gasgenerated from the reaction may be used in power applications such as infuel cells and internal combustion engines, and in propulsionapplications such as gas turbine engines.

In a preferred embodiment of the invention, a method of generatinghydrogen gas is provided which comprises providing stabilized aluminumnanoparticles which have been formed by the decomposition of an alaneprecursor in the presence of a catalyst and an organic or organometallicpassivating agent, and reacting the stabilized nanoparticles with water;where the reaction provides at least an 85% hydrogen yield. Morepreferably, the reaction results in at least a 95% hydrogen yield.

By “stabilized,” it is meant that the nanoparticles do not react in airor upon exposure to solvents such as nonpolar hydrocarbons includinghexane and toluene, heteroatom and halogenated hydrocarbons such astetrahydrofuran and chloroform, and polar solvents such as ethanol andmethanol, but that the nanoparticles will readily react with water toform hydrogen.

The stabilized aluminum nanoparticles used in the reaction preferablyhave a size ranging from about 2 nm to about 100 nm in one embodiment,and more preferably, from about 20 to about 65 nm in another embodiment.In one embodiment, the ratio of aluminum nanoparticles to water by massis about 0.04 to about 1.0.

In one embodiment, the alane precursor is selected fromdimethylethylamine alane and 1-methylpyrrolidone alane and the catalystpreferably comprises titanium (iv) isopropoxide. In one embodiment, thepassivation agent is selected from the group consisting of carboxylicacids, amines, alcohols, thiols, acyl halides, ketones, aldehydes,carbonates, esters, epoxides, organic metals, or combinations thereof.

In one preferred embodiment, the aluminum nanoparticles are formed bycombining an alane precursor, a catalyst; and an organic passivatingagent in solution; and sonicating the solution. In this embodiment, thealane precursor preferably comprises alane N,N-dimethylethylamine, thecatalyst preferably comprises titanium (IV) isopropoxide, and thepassivation agent is selected from the group consisting of carboxylicacids, amines, and alcohols. In a preferred embodiment, the passivationagent comprises oleic acid. In this embodiment, the formed nanoparticleshave a size ranging from about 5 to about 70 nm, and an average size ofabout 30 nm. The aluminum nanoparticles formed by this method compriseabout 35% of an organic shell layer (by mass).

The resulting stabilized aluminum nanoparticles are capable of reactingwith water and can be used to generate large amounts of hydrogen in aneasy and environmentally friendly process for use in power generation.

Accordingly, the various embodiments of the invention provide a methodof generating hydrogen gas from the reaction of stabilized aluminumnanoparticles with water and a method of synthesizing stabilizedaluminum nanoparticles having water reactivity. Other features andadvantages of the various embodiments of the invention will be apparentfrom the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a mass spectra illustrating hydrogen produced from thereaction of aluminum-oleic acid nanoparticles and water;

FIG. 2 is x-ray diffraction spectra for the aluminum-oleic acidnanoparticles and the product of the reaction of the nanoparticles withwater in a reaction with small quantities of water and in a reactionwith large quantities of water;

FIG. 3 is a graph plotting the temperature vs. time for the reaction ofthe aluminum-oleic acid nanoparticles with water;

FIG. 4 is a graph depicting the maximum temperature vs. mass ratio ofAlOA:water;

FIG. 5 is a graph illustrating pressure vs. time for the reaction ofaluminum-oleic acid nanoparticles with water and voltage and current vs.time for the operation of a fuel cell using the hydrogen produced fromthe reaction of aluminum-oleic acid nanoparticles with water;

FIG. 6 is a series of X-ray diffraction spectra for various aluminumorganic core-shell nanoparticles in comparison with the XRD diffractionpattern for aluminum; and

FIG. 7 illustrates X-ray diffraction spectra of aluminum-oleic acidnanoparticles before and after exposure to various solvents.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The reaction of stabilized aluminum nanoparticles with water to yieldhydrogen gas can be performed in a simple manner, requiring no promotersor initial energy to initiate the reaction. This reaction is achieved bythe synthesis of aluminum nanoparticles having stability in air andorganic solvents but which readily react with water. Nanoscale aluminumis preferred for use in the reaction as aluminum nanoparticlesdemonstrate enhanced reactivity, i.e., they oxidize more quickly andfully than micrometer-sized or bulk-sized aluminum due to their largespecific surface area and energy density. However, commercial aluminumnanoparticles having an aluminum oxide shell do not readily react withwater under ambient conditions (room temperature and standard pressure).The inventors have discovered that by chemically modifying the aluminumoxide shell of nanoparticles during synthesis to provide an organicshell, the nanoparticles can be reacted with water at room temperatureto generate hydrogen.

The reaction of the aluminum nanoparticles with water is a near completereaction, and the rate of hydrogen production can be varied bycontrolling the nanoparticle-to-water mass ratio. The hydrogen generatedby this reaction is sufficient to provide power without requiring thedirect storage of large quantities of hydrogen. Thus, hydrogen can begenerated with the addition of water where and when needed.

A number of methods may be used for producing stabilized aluminumnanoparticles for reactivity with water. The synthesis of stabilizedaluminum nanoparticles generally involves the thermal decomposition ofan alane precursor in the presence of a titanium catalyst and an organicpassivation agent. The preferred catalyst is titanium (IV) isopropoxide.

Generally, any alane or alane complex is suitable for use in the method.Preferred alanes for use include dimethylethylamine alane,H₃AlN[(CH₃)₂C2H₅] and 1-methylpyrrolidine alane, H₃AlN[(C₄H₈)(CH₃)].

Suitable organic or organometallic passivation agents include carboxylicacids, amines, alcohols, thiols, acyl halides, ketones, aldehydes,carbonates, esters, epoxides and organic metals. Examples of specificpassivation agents include oleic acid, 2-hexadecanone, 1-octanol, Span80, dodcecyl aldehyde, 1,2-epoxydodecane, arachidyl dodecanoate,octadecylamine, chloro(dimethyl)octdecylsilane, and nickel stearate. Theorganic passivation agent functions to cap the growing nanoparticlesurface to stop particle growth, keeping the aluminum particles to ananoscale size during their formation and during alane decomposition andalso protects the formed nanoparticles from oxidizing at the surface.While not wishing to be bound by theory, it is believed that the organicpassivation agent used to form the nanoparticles provides an organicoxide layer or core shell on the nanoparticle which provides the desiredparticle stability, i.e., stability in air and solvents but reactivitywith water.

It should be appreciated that the concentration of the passivation agentaffects the aluminum nanoparticle core size; i.e., a higherconcentration of passivation agent results in a smaller nanoparticlesize, while a lower concentration results in a larger particle size. Forexample, when a high concentration of passivation agent is used, theparticle size is about 5 nm. This effect is caused by the balancebetween the number of molecules available to cap a growing nanoparticlesurface and the amount of surface area of the growing nanoparticle. As ananoparticle grows larger, the surface area decreases. This growthoccurs until the amount of passivation agent is sufficient to cap thatsurface. Less passivation agent will cover less surface area, thus theparticles will grow larger before they are capped.

A preferred method of synthesizing aluminum nanoparticles for reactionwith water to generate hydrogen is the sonochemically-assisted thermaldecomposition of alane. In one preferred method, N,N-dimethylethylaminealane is combined with a titanium isopropoxide catalyst and an organicpassivation agent comprising oleic acid. The synthesis is described inmore detail in K. A. Fernando et al., “Sonochemically Assisted ThermalDecomposition of Alane N,N-Dimethylethylamine with Titanium (IV)Isopropoxide in the Presence of Oleic Acid to Yield Air-Stable andSize-Selective Aluminum Core-Shell Nanoparticles,” J. Phys. Chem. C.,Vol. 113, No. 2, 2009, pp. 500-503, the disclosure of which isincorporated herein by reference. A solution of oleic acid is preparedwith a solvent such as dodecane and combined with the alane and titanium(iv) isopropoxide catalyst. The resulting solution is then sonicated forabout 7.5 minutes at an amplitude of about 37%, (i.e., the percentage ofthe sonicator head's incursion into the solution) and a power use ofabout 22 W to produce a solution that gradually precipitates and yieldsa powder which can be recovered by evaporation of the solvent undervacuum.

The aluminum oleic-acid nanoparticles formed from this process have aface-centered cubic crystal (fcc) structure comprising an inner aluminumcore surrounded by an oxide shell and an outer organic shell. Thestructure comprises about 40% aluminum core, about 35% organic shell(oleic acid), and about 25% inner oxide shell by mass based on the totalparticle mass. The nanoparticles range in size from about 2 nm to about100 nm, and have an average size of about 30 nm. However, it should beappreciated that larger size nanoparticles (greater than 100 nm) mayalso be used.

It should also be noted that while oleic acid is a preferred cappingagent for the sonochemical thermal decomposition method, other organiccapping agents may be used, including 2-hexadecanone, 1-octanol, Span80, dodecyl aldehyde, 1,2-epoxydodecane, arachidyl dodecanoate,octadecylamine, chloro(dimethyl)octdecylsilane, and nickel stearate.

In studying the nature of the metal-organic bonding in the formedaluminum-oleic acid core-shell nanoparticles, the inventors have foundthat the organic layer on the nanoparticles appears to be attached viaAl—O—C bonds with the C atom formerly involved in the carboxylic acidfunctional group. IR spectra of the nanoparticles indicates that C—H andO—H groups are present but that carbonyl and carboxylate signals aresubstantially absent. In addition, X-ray photoelectron spectroscopy(XPS) testing has confirmed the presence Al—O and/or Al—O—C bonds, butshows no Al—C bonds. Thus, it is believed that the oleic acidpassivation agent is bound neither as a carboxylate nor via Al—C bondsresulting from decomposed carboxylates, but rather appears to beattached via Al—O—C bonds with the C atom formerly involved in thecarboxylic acid functional group. See Lewis et al. “Multispectroscopic(FTIR, XPS, and TOFMS-TPD) Investigation of the Core-Shell Bonding inSonochemically Prepared Aluminum Nanoparticles Capped with Oleic Acid,”J. Phys. Chem., vol. 114, no. 14, 6377-6380, 2010, the disclosure ofwhich is incorporated herein by reference.

The oxide shell of the aluminum nanoparticle is believed to be formedfrom oxygen atoms brought to the aluminum surface by the oleic acidpassivation agent, i.e., as the passivation agent attaches to thealuminum nanoparticle surface as part of the particle growth process,the oxygen from the carboxylic acid functional group (RCOO—) bonds tothe aluminum surface, thus providing the oxygen needed to form theprotective oxide shell. It is also believed that the oleic aciddecomposes as it reacts to passivate the aluminum nanoparticle surface,i.e., bonds are broken with the carboxylic acid functional group; thus,the oleic acid molecule is no longer technically oleic acid but is nowessentially part of the nanoparticle molecule.

Thermal analysis of stabilized aluminum nanoparticles produced by thesonochemical method has shown that the oxide shell does not behave inthe same way as the oxide shell of natural aluminum oxide nanoparticles,reacting at much lower temperatures (about 150 to 400° C.) in comparisonwill standard aluminum nanoparticles, which react/oxidize at elevatedtemperatures of about 500° C. to 600° C. Further, it is noted thatnanoparticles prepared by conventional thermal decomposition of alane donot exhibit the stability achieved using sonochemistry, i.e., theyoxidize within minutes to days. In contrast, the oxide shell of thealuminum-oleic acid nanoparticles formed by the sonochemical thermaldecomposition method is believed to prevent oxidation of the aluminummetal under ambient conditions, i.e., it does not readily react with airor moisture in the air. The inventors have found that samples stored inglass bottles on a countertop remained viable after several months ofstorage with no special precautions.

While not wishing to be bound by theory, it is believed that there areunique aspects of the sonochemical process which influence theproperties of the nanoparticles; for example, sonochemistry generatesextreme temperature regions (micron sized bubbles) in the reactionsolution that can reach temperatures as high as 5,000 K with very rapidcooling rates (about 10¹⁰ K/sec), which then promotes radical formation,and displays the phenomenon of jetting (harsh impacting of material onthe micron scale).

The subsequent reaction of the formed aluminum-oleic acid nanoparticleswith water at room temperature is nearly complete, with hydrogen gasyields of 85% to 95% of the theoretical yield. This reaction is shownbelow and corresponds to earlier Equation (2):

2Al+4H₂O→2AlO(OH)+3H₂  (2)

where AlO(OH) is boehmite and where 2 moles of aluminum are expected toyield 3 moles of molecular hydrogen gas. The boehmite byproduct can berecycled or discarded. The rate of hydrogen production may be controlledby adjusting the amount of water.

In a typical reaction which corresponds to equation (2) above, about 1gram of aluminum-oleic acid nanoparticle material is placed within a 25mL stainless-steel pressure vessel, and about 2 mL of room temperaturetap water is added. The vessel is immediately sealed. The pressuretypically reaches about 400 psi in less than about 30 seconds, and thetemperature of the vessel reaches about 130° C. in about the same time.Thus, the reaction is rapid and may be completed within several minutes.One (1) gram of aluminum-oleic acid nanoparticles (AlOA) producesapproximately 0.5 L of hydrogen gas.

The nanoparticle-water reaction is highly exothermic due to high heat ofreaction of aluminum metal with water, resulting in significant localheating. The exothermic properties and self-heating nature of thereaction allow the rate of the reaction to be controlled throughmanipulation of the system temperature. While the reaction may beinitiated at room temperature and standard pressure, the use of highertemperatures will make the reaction proceed faster, while lowertemperatures will slow the reaction. For example, if active cooling isapplied, the hydrogen may be able to be slowly delivered as it isproduced, negating the need for a high pressure vessel and improving thesafety of the system.

The reaction is strongly influenced by the mass ratio of aluminumnanoparticles to water, which may vary from 0.04 to 1.0. The inventorshave found that the rate of hydrogen production can be controlled bycontrolling this nanoparticle-to-water mass ratio. A desired rate ofhydrogen production is between about 6.4×10⁻⁴ and 0.01 g of H₂/s/g ofaluminum. See Bunker et al., “Spontaneous Hydrogen Generation fromOrganic-Capped Al Nanoparticles and Water,” Applied Materials &Interfaces, Vol. 2, No. 1, 11-14, 2010, the disclosure of which isincorporated herein by reference.

While the sonochemical method of synthesizing aluminum nanoparticles hasbeen primarily described herein, it should be appreciated that othermethods may be used to produce stable aluminum nanoparticles which arereactive with water to produce hydrogen.

One suitable method is a wet chemical synthesis of aluminumnanoparticles utilizing alane precursors including dimethylethylaminealane and 1-methylpyrrolidone alane. The passivation agents may compriseC₁₃F₂₇COOH, 1H,1H-perfluoro-1-tetradecanol (C₁₃F₂₇CH₂OH), tetradecanoicacid (C₁₃H₂₇COOH), HDIPA, and octadecylamine, or combinations thereof.This method of synthesis is described in Meziani et al., “Formation andProperties of Stabilized Aluminum Nanoparticles,” Applied Materials &Interfaces, Vol. 1, No. 3, 703-709, 2009, the disclosure of which isincorporated herein by reference. The method includes combining thealane precursors in solution, adding a titanium isopropoxide catalyst,and adding the passivation agent(s), followed by removal of residualsolvent.

In another embodiment, the aluminum nanoparticles are formed in apolymerization encapsulation method by combining an alane precursor, acatalyst; and a passivating agent comprising an epoxide selected fromepoxyhexane and epoxydodecane. In this method, a solution ofN,N-dimethylethylamine alane in toluene is combined with a titaniumisopropoxide solution and a stoichiometric amount of epoxide is added asthe passivation agent. The reaction is stirred for about 30 minutesfollowed by removal of the solvent. The epoxides polymerize on thenanoparticle to produce a polyether such that the aluminum nanoparticlecore is surrounded by a protective oxygen-rich polyether cap. See Chunget al., “Capping and Passivation of Aluminum Nanoparticles UsingAlkyl-substituted Epoxides,” Langmuir 2009, 25 (16), 8883-8887, thedisclosure of which is incorporated herein by reference. In this method,the stabilized aluminum nanoparticles have a size of from about 20 toabout 30 nm.

The inventors have also found that the use of organometallic passivationagents such as silicon and nickel are effective in forming stabilizednanoparticles capable of reacting with water using the sonochemicalthermal decomposition method. The use of aluminum silicon core-shellnanoparticles is preferred as they provide the highest aluminum contentand hydrogen yield.

Table 1 below illustrates a number of capping/passivation agents whichcan be used to synthesize aluminum nanoparticles using the sonochemicalthermal decomposition method, along with the resulting nanoparticlesize, shell content, and aluminum content.

TABLE 1 Al-CAPPING AGENT Core-Shell Nanoparticles and PropertiesFunctional ^(b)Shell ^(c)Al Capping Agent group ^(a)NP Size contentcontent Oleic Acid carboxylic acid 60 nm 35% 40% 2-Hexadecanone ketone45 nm 24% 43% 1-octanol alcohol 50 nm 30% 42% Span 80 multi 45 nm 50%37% dodecyl aldehyde aldehyde 40 nm 21% 34% 1,2-epoxydodecane epoxide 20nm 33% 25% arachidyl dodecanoate ester 45 nm 45% 20% octadecylamineamine 30 nm 25% 18% chloro(dimethyl)octdec- silane 50 nm 31% 50%ylsilane nickel stearate nickel/ 65 nm 38% 20% carboxylate^(a)nanoparticle size estimated from XRD analysis ^(b)shell contentdetermined from TGA analysis ^(c)Al content determined from quantitativehydrogen yield using AlOA as standard, AlOA determined from ICP-MSanalysis

FIG. 6 illustrates X-ray diffraction (XRD) spectra of various aluminumorganic core-shell nanoparticles formed using the above passivationagents in comparison with the XRD diffraction pattern for face-centeredcubic aluminum. As can be seen, the peaks for the nanoparticlescorrespond to the peaks for aluminum, confirming the ability to formaluminum nanoparticles with an aluminum core.

In yet another embodiment, the stabilized aluminum nanoparticles areformed by a polymer impregnation and encapsulation method in which analane precursor is combined with a polymer membrane in solution. Thepolymer membrane preferably comprises a perfluorinated ion-exchangemembrane. In this embodiment, the resulting stabilized nanoparticleshave a size of from about 10 to about 15 nm. In the method, a1-methylpyrrolidine alane precursor is provided in solution and combinedwith a polymer membrane comprising Nation-117 membrane film(commercially available under the designation DuPont™ Nafion®) which hasbeen soaked in an isopropanol solution. See Li et al., “TemplatedSynthesis of Aluminum Nanoparticles—A New Route to Stable EnergeticMaterials,” J. Phys. Chem. C, Vol. 113, No. 48, 2009, the disclosure ofwhich is incorporated herein by reference. The nanoparticles becomeembedded in the membrane structure, which protects the Al nanoparticlesfrom significant oxidation, making them stable in ambient air.

It should be appreciated that any method of synthesizing the aluminumnanoparticles may be used as long as the resulting nanoparticlesdemonstrate the required air and solvent stability and reactivity withwater to produce hydrogen. In addition to the methods described above,laser ablation, evaporation/condensation, and sono-electrochemicalmethods may be used to form the aluminum nanoparticles. See, forexample, Eliezer et al., Synthesis of nanoparticles with femtosecondlaser pulses, Physical Review B 69, 144119 (2004), Normatov et al.,Preparation of Benzene-Stabilized Aluminum Nanoparticles in HydrogenAtmosphere, Inorganic Materials Vol. 37, No. 10, 2001, pp. 1002-1005,and Mahendiran et al., “Sonoelectrochemical Synthesis of MetallicAluminum Nanoparticles, Eur. J. Inorg. Chem. 2009, 2050-2053.

In order that the invention may be more readily understood, reference ismade to the following examples which are intended to illustrate theinvention, but not limit the scope thereof.

Example 1

Aluminum nanoparticles (aluminum-oleic acid core-shell nanoparticles)were formed in accordance with the invention by sonochemically-assistedthermal decomposition of alane. The materials used included alaneN,N-dimethylethylamine in a 0.4 M toluene solution, titanium (IV)isopropoxide (98%), oleic acid (99%), and dodecane (99% anhydrous), allobtained from Aldrich Chemicals. Hexane (Optima grade) was obtained fromFisher Scientific.

4 mL of a deoxygenated solution of oleic acid in dodecane (0.058 M) wascombined with 10 mL of a solution of alane N,N-dimethylethylamine intoluene (0.5 M), making a 14 mL working solution with an oleic acidconcentration of 0.017 M and an alane concentration of 0.36 M. To thiscombined solution was added 15.5 μL of titanium (IV) isopropoxide,resulting in a concentration of 3.7×10⁻³ M. The working solution wasthen sonicated for 7.5 minutes active time at an amplitude of about 37%and a power use of about 22 W following a one-second-on, one-second-offprocedure (duty cycle about 46%). These conditions produced a blackcolored solution that gradually precipitated, yielding a grayish-blackpowder. The powder was recovered by evaporation of the solvent undervacuum followed by repeated washings with hexane. The reaction yieldedbetween 200 and 250 mg aluminum nanoparticles per synthesis.

Example 2

To demonstrate the stability of aluminum-oleic acid nanoparticles formedin accordance with Example 1, aliquots of about 20 mg of aluminumnanoparticle samples were suspended in a series of solvents (5 mL) andagitated in a simple sonic bath for 90 minutes. The solvents includedhexane, toluene, tetrahydrofuran (THW), chloroform, ethanol, methanol,and water. In all cases but one (water), the particles remainedunchanged as measured by powder x-ray diffraction. Only water showed achange in the particles, evidenced by a significantly altered x-rayspectrum as shown in FIG. 7. Also as shown, (A) represents the spectrafor the as-synthesized aluminum oleic acid nanoparticles, (B) representsthe nanoparticles after exposure to methanol and is representative ofexposure to hexane, toluene, THF, chloroform, and ethanol, (C)represents the product of the reaction of the aluminum nanoparticleswith water at an AlOA:H₂O ratio of 10⁻³; and (D) represents the productof the reaction of the aluminum nanoparticles with water at an AlOA:H₂Oratio of 0.5.

Example 3

To test the reaction of aluminum-oleic acid nanoparticles with water, asmall quantity of nanoparticle sample (20 mg) obtained from Example 1was mixed with water (5 mL) and the headspace sampled with ahome-assembled quadrupole mass spectrometer. After subtraction of abackground spectrum, the data showed a strong signal for hydrogen gas asshown in FIG. 1. It should be noted in that in these experiments, thealuminum-oleic acid nanoparticle (AlOA) to water mass ratio was fairlysmall (AlOA:H₂O=10⁻³). When performed at a much larger ratio (e.g.,0.5), the reaction appeared far more vigorous and generated considerableheat. The x-ray spectrum of oxide product formed under those conditionsdemonstrated a clear pattern for boehmite (AlOOH) as shown in FIG. 2 topidentified as face-centered cubic aluminum, middle identified asboehmite (product of the reaction of the nanoparticles with water in areaction with small quantity of water) and bottom identified as amixture of aluminum oxides with boehmite (reaction with large quantityof water)

To further investigate these observations, a small-scale temperaturemeasurement system was assembled using a thermocouple affixed to analuminum cup. Water (a constant 60 μL) was added to AlOA samples varyingin mass from about 2 mg to 15 mg (AlOA:H₂O=0.03 to 0.25). Temperaturewas recorded as a function of time, and the data was plotted as shown inFIG. 3. The traces of temperature vs. time were all similar in that theyexhibited an induction phase, a rapid rise, a maximum, and then a steadydecrease, eventually returning to room temperature. A plot of the slopesobtained from the rapid rise section of each trace vs. sample mass isalso shown in FIG. 3. The data indicate that as mass increases, the rateat which the sample reaches its maximum temperature increasesnon-linearly. This observation is indicative of a self-heating reaction;as the sample reacts, the heat generated accelerates the reaction. Thiseffect is amplified as the sample mass increases. Also as shown in FIG.3 are the values for maximum temperature vs. sample mass. The plot islinear, indicating that the reaction yield is constant. FIG. 4 furtherillustrates the maximum temperature vs. AlOA:water mass ratio and thechange in temperature per change in time for the rapid rise section ofthe curves vs. mass ratio.

To obtain a value for the yield, the same basic experiment was performedusing a 25 mL pressure vessel coupled to a digital pressure meter. 1 gof sample and 2 mL of water were added to the vessel (AlOA:H₂O=0.05),resulting in a rapid increase in pressure due to hydrogen gasgeneration. The pressure was plotted vs. time as shown in FIG. 5 andshows a rapid rise that then slowed to reach a plateau. The pressure atthe plateau was 309 psi, or 21 atm. Based on the composition of thesamples (about 40% aluminum metal), using the stoichiometry of2Al+4H₂O→2Al(OH)+3H₂, and applying the ideal gas law, an approximate 95%yield was calculated for the formation of hydrogen gas. The rate atwhich hydrogen was generated under the continuous reaction conditionswas about 0.01 g of H₂/s/g of aluminum.

The utility of the hydrogen generated is shown in FIG. 5 where justafter the plateau was reached, the pressure vessel was placed in-linewith a fuel cell equipped with a pressure regulator set to deliver lessthan about 5 psi hydrogen. A simple computer fan was attached to thefuel cell to serve as the electrical load, and the voltage and currentwere recorded as the hydrogen was consumed. Once the hydrogen wasdelivered to the cell, the voltage and current quickly reached stableworking values (about 13 V and 0.15 A). The power consumed by thissystem is about 2 W for a continuous 2.3 minutes.

Thus, with the method described herein, hydrogen can be used as a viablefuel in a number of energy conversion devices. When hydrogen is coupledwith a fuel cell, it may be used to power communications equipment suchas radios and cell phones, computers, and lighting. Other potentialapplications include remote sensors, vehicles, and combustionapplications such as micro turbines and thermoelectric engines. Themethod provides an advantage in that aluminum and water can be stored togenerate hydrogen where and when needed, without the need for storingand/or transporting large quantities of gaseous or liquid hydrogen,which can be dangerous.

Having described the invention in detail and by reference to preferredembodiments thereof, it will be apparent that modifications andvariations are possible without departing from the scope of theinvention.

What is claimed is:
 1. A method of generating hydrogen gas comprising:providing stabilized aluminum nanoparticles formed by the decompositionof an alane precursor in the presence of a catalyst and an organic ororganometallic passivating agent; and reacting the nanoparticles withwater; wherein the reaction provides a hydrogen yield of at least 85%.2. The method of claim 1, wherein the hydrogen yield is at least a 95%.3. The method of claim 1, wherein the nanoparticles have a particle sizefrom about 5 nm to about 70 nm.
 4. The method of claim 1, wherein theratio of aluminum nanoparticles to water by mass is about 0.04 to about1.0.
 5. The method of claim 1, wherein the alane precursor is selectedfrom dimethylethylamine alane and 1-methylpyrrolidone alane.
 6. Themethod of claim 1, wherein the catalyst comprises titanium (iv)isopropoxide.
 7. The method of claim 1, wherein the passivation agent isselected from the group consisting of carboxylic acids, amines,alcohols, thiols, acyl halides, ketones, aldehydes, carbonates, esters,epoxides, organic metals, or combinations thereof.
 8. The method ofclaim 7, wherein the passivation agent comprises an epoxide selectedfrom epoxyhexane and epoxydodecane.
 9. The method of claim 8, whereinthe aluminum nanoparticles have a size of from about 20 nm to about 30nm.
 10. The method of claim 1, wherein the aluminum nanoparticles have aface-centered cubic structure comprising an inner aluminum coresurrounded by an oxide shell and an organic outer shell.
 11. The methodof claim 10, wherein the organic outer shell comprises oleic acid. 12.The method of claim 11, wherein the aluminum nanoparticles are about 40%by weight inner aluminum core, about 35% by weight organic shell, andabout 25% by weight oxide shell, based on the total weight of thealuminum nanoparticles.